Is there a way to see thumbnails of all the materials in a scene?
What you really want is an old todo item. In two ways things already possible though.


Browse the materials in memory for linking (menu button to left of button where you can change material name). It shows icon-size previews. Blender already supports popups with larger previews (used for matcaps, brushes). Something similar could be coded to be used here too.
You can inspect materials (with previews) inside any other .blend file with using the File->Append browser, it allows to go into a file and see materials. Set file browser to view thumbnails. This type of browsing could be used for linked large collections of materials, textures or images too.